Workflow
We first load the packages needed for the analysis.
Prepare the required datasets
softwareRisk draws on the representation of the model’s
source code as a directed call graph \(G =
(V,E)\) in which each node \(v_i \in
V\) is a function or subroutine and each directed edge \(e_{ij} = (v_i, v_j) \in E\) is a function
call. It also assumes that each function will have a cyclomatic
complexity value. Therefore the analyst should have two different
datasets before starting the analysis:
A spreadsheet listing the set set of directed edges as an edge list, with one row per function call. The first column may contian the caller function (source node, \(v_i\), “from”) and the second column the calle function (target node, \(v_j\), “to”):
A spreadsheet listing the cyclomatic_complexity values for each function in the model.
Let us create these datasets to illustrate their format:
# Dataset 1: calls (edge list) -------------------------------------------------
calls_df <- data.frame(
from = c("clean_data", "compute_risk", "compute_risk", "calc_scores", "plot_results"),
to = c("load_data", "clean_data", "calc_scores", "mean", "compute_risk")
)
calls_df
#> from to
#> 1 clean_data load_data
#> 2 compute_risk clean_data
#> 3 compute_risk calc_scores
#> 4 calc_scores mean
#> 5 plot_results compute_risk
# Dataset 2: cyclomatic complexity (node attributes) ---------------------------
cyclo_df <- data.frame(
name = c("clean_data", "load_data", "compute_risk", "calc_scores", "mean", "plot_results"),
cyclo = c(6, 3, 12, 5, 2, 4)
)
cyclo_df
#> name cyclo
#> 1 clean_data 6
#> 2 load_data 3
#> 3 compute_risk 12
#> 4 calc_scores 5
#> 5 mean 2
#> 6 plot_results 4The analyst can then merge them into a tbl_graph:
# Merge into a tbl_graph -------------------------------------------------------
graph <- tbl_graph(nodes = cyclo_df, edges = calls_df, directed = TRUE)
graph
#> # A tbl_graph: 6 nodes and 5 edges
#> #
#> # A rooted tree
#> #
#> # Node Data: 6 × 2 (active)
#> name cyclo
#> <chr> <dbl>
#> 1 clean_data 6
#> 2 load_data 3
#> 3 compute_risk 12
#> 4 calc_scores 5
#> 5 mean 2
#> 6 plot_results 4
#> #
#> # Edge Data: 5 × 2
#> from to
#> <int> <int>
#> 1 1 2
#> 2 3 1
#> 3 3 4
#> # ℹ 2 more rowsOnce this is done, the data is prepared for
softwareRisk.
Analysis
Here we illustrate the functions of softwareRisk by
using the build-in data synthetic_graph. It consists of
five entry nodes, 35 middle nodes and 15 sink nodes. Each entry node
calls between two and five middle nodes and each middle node calls one
to three sink nodes, thus simulating realistic code architecture. The
synthetic example reproduces the characteristic right-tailed
distribution of cyclomatic complexity found in real software systems,
with many low-complexity functions and few highly complex ones (Landman et al. 2016).
# Load the data ----------------------------------------------------------------
data("synthetic_graph")
# Print it ---------------------------------------------------------------------
synthetic_graph
#> # A tbl_graph: 55 nodes and 122 edges
#> #
#> # A directed acyclic simple graph with 1 component
#> #
#> # Node Data: 55 × 2 (active)
#> name cyclo
#> <chr> <dbl>
#> 1 E1 7
#> 2 M15 10
#> 3 M14 39
#> 4 M3 12
#> 5 M10 13
#> 6 E2 43
#> 7 M25 17
#> 8 M26 3
#> 9 E3 6
#> 10 M5 8
#> # ℹ 45 more rows
#> #
#> # Edge Data: 122 × 2
#> from to
#> <int> <int>
#> 1 1 2
#> 2 1 3
#> 3 1 4
#> # ℹ 119 more rowsThe next step is to compute the in-degree and betweenness centrality
of each node, calculate its risk score and identify all simple paths
through the directed function-call graph. All this is done with the
all_paths_fun function. The in-degree and betweenness of
the nodes are calculated internally by functions in the
igraph package.
Definition of software risk
Risk is computed using a weighted power-mean aggregation of normalized cyclomatic complexity, in-degree and betweenness. The power parameter \(p\) controls how attributes combine: values of \(p < 1\) emphasize functions where several risk factors co-occur, while values of \(p > 1\) increasingly focus on the largest individual contributor. When \(p = 1\), the formula reduces to a simple weighted sum (additive risk).
\[\begin{equation} r^{(p)}_{(v_i)} = \left( \alpha\, \tilde{C}_{(v_i)}^{p} + \beta\, \tilde{d}_{(v_i)}^{\mathrm{in}\,p} + \gamma\, \tilde{b}_{(v_i)}^{p} \right)^{1/p}, \qquad p \in [-1,2]. \label{eq:composite_risk_score_power} \end{equation}\]
where the tilde \(\tilde{}\) refers to normalization, \(C\) denotes cyclomatic complexity, \(d^{\text{in}}\) refers to in-degree and \(b\) denotes betweenness. The weights \(\alpha\), \(\beta\) and \(\gamma\) reflect the relative importance of complexity, coupling and network position and can be defined by the analyst, with the constraint that \(\alpha+\beta+\gamma =1\). High \(r\) values indicate functions that are complex and/or highly interconnected and hence potential points of structural vulnerability.
Path-level risk scores
The risk scores computed at the function level are then aggregated at the path level as
\[\begin{equation} P_k = 1 - \prod_{i=1}^{n_k} (1 - r_{k(v_i)})\,, \label{eq:independent_events} \end{equation}\]
where \(r_{k(v_i)}\) is the risk of the \(i\)-th function in path \(k\) and \(n_k\) is the number of functions in that path. \(P_k\) is at least as large as the maximum individual function risk and monotonically increases as more functions on the path become risky, approaching 1 when several functions have high risk scores. High \(P_k\) scores thus identify not only vulnerable paths, but also paths whose potential failure can have a larger cascading effect into other parts of the system through their shared high-centrality functions.
In this example, we set \(p=1\) (additive risk), \(\alpha=0.6\), \(\beta=0.3\) and \(\gamma = 0.1\), thus prioritizing defect-proneness and the likelihood of unexpected behaviours and relegating propagation potential as secondary.
# Run the function -------------------------------------------------------------
output <- all_paths_fun(graph = synthetic_graph,
alpha = 0.6,
beta = 0.3,
gamma = 0.1,
complexity_col = "cyclo",
weight_tol = 1e-8)
# Print the output -------------------------------------------------------------
output
#> $nodes
#> # A tibble: 55 × 6
#> name cyclomatic_complexity indeg outdeg btw risk_score
#> <chr> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 E1 7 0 4 0 0.0610
#> 2 M15 10 3 3 14.5 0.236
#> 3 M14 39 2 3 12 0.485
#> 4 M3 12 1 3 2.5 0.157
#> 5 M10 13 3 2 34.8 0.289
#> 6 E2 43 0 3 0 0.427
#> 7 M25 17 3 4 25 0.319
#> 8 M26 3 2 3 7 0.114
#> 9 E3 6 0 4 0 0.0508
#> 10 M5 8 1 6 7.25 0.122
#> # ℹ 45 more rows
#>
#> $paths
#> # A tibble: 209 × 10
#> path_id path_nodes path_str hops path_risk_score path_cc gini_node_risk
#> <int> <list> <chr> <dbl> <dbl> <list> <dbl>
#> 1 1 <chr [6]> E1 → M14 → M… 5 0.900 <dbl> 0.253
#> 2 2 <chr [4]> E1 → M14 → M… 3 0.793 <dbl> 0.277
#> 3 3 <chr [5]> E1 → M10 → M… 4 0.773 <dbl> 0.243
#> 4 4 <chr [6]> E2 → M14 → M… 5 0.939 <dbl> 0.136
#> 5 5 <chr [4]> E2 → M14 → M… 3 0.874 <dbl> 0.0946
#> 6 6 <chr [5]> E2 → M25 → M… 4 0.868 <dbl> 0.132
#> 7 7 <chr [3]> E2 → M25 → S… 2 0.725 <dbl> 0.0841
#> 8 8 <chr [5]> E3 → M25 → M… 4 0.781 <dbl> 0.253
#> 9 9 <chr [3]> E3 → M25 → S… 2 0.545 <dbl> 0.269
#> 10 10 <chr [6]> E3 → M5 → M1… 5 0.799 <dbl> 0.285
#> # ℹ 199 more rows
#> # ℹ 3 more variables: risk_slope <dbl>, risk_mean <dbl>, risk_sum <dbl>The output of all_paths_fun is a list with two slots,
nodes and paths.
$nodes: it contains the name of the nodes and their relevant metrics (cyclomatic complexity, in-degree, out-degree, betweenness and risk score).$paths: it describes each simple path in the call graph and includes the ordered sequence of nodes forming the path, the number of hops (i.e., the number of edges traversed along the path), the path-level risk score and the vector of cyclomatic complexity values for the nodes along the path (path_cccolumn). In addition, two distributional metrics are reported: the Gini coefficient of node-level risk along the path (gini_node_riskcolumn), which quantifies the inequality of risk contributions across nodes within a path, and the risk_slope, which captures the direction and magnitude of change in node-level risk from the beginning to the end of the path.
Plotting
softwareRisk provides functions to inspect the results
of the analysis. The function path_fix_heatmap allows the
analyst to chose the top \(n\) nodes
and \(n\) paths in terms of their risk
score and observe how much the risk score of the riskiest paths would
decrease if the selected high-risk nodes were made perfectly reliable.
This analysis identifies nodes that act as chokepoints for risk
propagation, highlights paths dominated by single high-risk functions
and reveals which refactoring actions would yield the greatest
reductions in path-level risk.
path_fix_heatmap(all_paths_out = output, n_nodes = 20, k_paths = 20)
#> $delta_tbl
#> # A tibble: 400 × 3
#> node path_id deltaR
#> <fct> <fct> <dbl>
#> 1 S6 44 0
#> 2 S6 146 0
#> 3 S6 192 0
#> 4 S6 103 0
#> 5 S6 39 0
#> 6 S6 57 0
#> 7 S6 143 0
#> 8 S6 159 0
#> 9 S6 93 0.260
#> 10 S6 4 0
#> # ℹ 390 more rows
#>
#> $plot
softwareRisk also allows to plot the call graph with the
top risky paths highlighted. This is done with the function
plot_top_paths_fun. The top ten most risky paths are
highlighted in colour. The thickness of the edge shows how frequently an
edge participates in the top 10 most risky paths. The color of the edge
(from orange to red) indicates the mean risk of paths including that
edge.
plot_output <- plot_top_paths_fun(graph = synthetic_graph,
all_paths_out = output,
model.name = "ToyModel",
language = "Fortran",
top_n = 10,
alpha_non_top = 0.05)
The color of the nodes maps onto the cyclomatic complexity categories defined by Watson and McCabe (1996) (0-10 low risk; 10-20 moderate complexity, 20-50 complex, high risk; \(>50\) very complex, untestable).
The alpha_non_top argument controls the transparency of
the paths that are not identified as top. For small or sparse models, it
can be set to alpha_non_top = 1 to better visualize the
full call graph:
plot_output <- plot_top_paths_fun(graph = synthetic_graph,
all_paths_out = output,
model.name = "ToyModel",
language = "Fortran",
top_n = 10,
alpha_non_top = 1)
Uncertainty and sensitivity analysis
softwareRisk also enables the analyst to perform
uncertainty and sensitivity analyses of risk and path score calculations
by leveraging the sensobol package (Puy et al.
2022). By systematically varying the weights \((\alpha, \beta, \gamma)\) and the power
parameter \(p\), the package allows
users to evaluate how sensitive node- and path-level risk scores are to
different risk conceptualizations. This approach makes it possible to
assess the robustness of the identification of high-risk paths under
alternative definitions of risk.
Uncertainty and sensitivity analyses are implemented through the
function uncertainty_fun. The user needs to define the
order of the effects explored (first, second
or third).
Internally, uncertainty_fun builds a Sobol’ quasi-random
design over four independent \(U(0,1)\)
draws. Three of them (a_raw, b_raw,
c_raw) are normalised to sum to one, yielding the weights
\(\alpha\), \(\beta\) and \(\gamma\); the fourth (p_raw)
is mapped linearly to \(p \in [-1,
2]\). Independent uniform inputs are required by the quasi-random
sequence, hence the need for the raw draws; the sensitivity indices,
however, are attributed to the transformed parameters and labelled
alpha, beta, gamma and
p in the output, so the results are directly interpretable
in terms of the model parameters.
# Run uncertainty analysis -----------------------------------------------------
uncertainty_analysis <- uncertainty_fun(all_paths_out = output,
N = 2^10,
order = "first")
# Print the top five rows ------------------------------------------------------
lapply(uncertainty_analysis, function(x) head(x, 5))
#> $nodes
#> # A tibble: 5 × 3
#> name uncertainty_analysis sensitivity_analysis
#> <chr> <list> <list>
#> 1 E1 <dbl [1,024]> <sensobol>
#> 2 M15 <dbl [1,024]> <sensobol>
#> 3 M14 <dbl [1,024]> <sensobol>
#> 4 M3 <dbl [1,024]> <sensobol>
#> 5 M10 <dbl [1,024]> <sensobol>
#>
#> $paths
#> # A tibble: 5 × 6
#> path_id path_str hops uncertainty_analysis gini_index risk_trend
#> <int> <chr> <dbl> <list> <list> <list>
#> 1 1 E1 → M14 → M23 → M29… 5 <dbl [1,024]> <dbl> <dbl>
#> 2 2 E1 → M14 → M23 → S15 3 <dbl [1,024]> <dbl> <dbl>
#> 3 3 E1 → M10 → M29 → M31… 4 <dbl [1,024]> <dbl> <dbl>
#> 4 4 E2 → M14 → M23 → M29… 5 <dbl [1,024]> <dbl> <dbl>
#> 5 5 E2 → M14 → M23 → S15 3 <dbl [1,024]> <dbl> <dbl>The output is a list with two slots:
$nodes: anamecolumn with the name of the node, anuncertainty_analysiscolumn with a vector of \(N\) node-level risk scores after randomizing \(\alpha\), \(\beta\), \(\gamma\) and \(p\), and asensitivity_analysiscolumn with the results of the sensitivity analysis. Each element ofsensitivity_analysisis asensobol::sobol_indices()object whose$resultsdata frame reports first-order (\(S_i\)) and/or total-order (\(T_i\)) indices for the four parameters, labelledalpha,beta,gammaandp. See thesensobolpackage for further details (Puy et al. 2022).$paths: apath_idcolumn with the ID number of the path, apath_strcolumn informing on the sequence of functions calls for that path, ahopscolumn informing on the number of edges traversed along the path and three columns informing on the results of the uncertainty analysis (UA):uncertainty_analysis: vector of path-level risk scores after the UA.gini_index: vector of gini_index values after the UA.risk_trend: vector of risk_slope values after the UA.
The analyst can also plot the top \(n\) risky paths and their uncertainty with
the function path_uncertainty_plot. The error bars
encompass the minimum, mean and maximum \(P_k\) value for that path after the
uncertainty analysis.
path_uncertainty_plot(ua_sa_out = uncertainty_analysis, n_paths = 20)
#> `height` was translated to `width`.
Extracting and plotting Sobol’ indices
The sensitivity_analysis list-column in
$nodes stores one sensobol::sobol_indices()
object per node. The Sobol’ indices for a given node are accessible via
the $results slot of that object, which is a data frame
with three columns: original (the index value),
sensitivity ("Si" for first-order,
"Ti" for total-order), and parameters
("alpha", "beta", "gamma",
"p").
# Sobol' indices for the first node
si_node1 <- uncertainty_analysis$nodes$sensitivity_analysis[[1]]$results
si_node1
#> Index: <sensitivity>
#> original sensitivity parameters
#> <num> <char> <char>
#> 1: 0.10120246 Si alpha
#> 2: 0.02898618 Si beta
#> 3: 0.02429100 Si gamma
#> 4: 0.71862368 Si p
#> 5: 0.19167436 Ti alpha
#> 6: 0.05858657 Ti beta
#> 7: 0.05915424 Ti gamma
#> 8: 0.83929541 Ti pTo compare parameter importance across all nodes, combine the per-node results into a single data frame:
sa_all <- do.call(rbind, Map(
function(sa, nm) data.frame(sa$results, name = nm, stringsAsFactors = FALSE),
uncertainty_analysis$nodes$sensitivity_analysis,
uncertainty_analysis$nodes$name
))
head(sa_all)
#> original sensitivity parameters name
#> 1 0.10120246 Si alpha E1
#> 2 0.02898618 Si beta E1
#> 3 0.02429100 Si gamma E1
#> 4 0.71862368 Si p E1
#> 5 0.19167436 Ti alpha E1
#> 6 0.05858657 Ti beta E1The resulting data frame can be used to visualize how much of the variance in node risk scores is explained by each parameter, and whether the dominant driver is consistent across nodes or varies. The plot below shows boxplots of \(S_i\) and \(T_i\) for each parameter across all nodes. A large gap between \(S_i\) and \(T_i\) for a parameter indicates important higher-order interactions with the other parameters.
library(ggplot2)
ggplot(sa_all, aes(x = parameters, y = original, fill = sensitivity)) +
geom_boxplot(alpha = 0.7) +
scale_fill_manual(
values = c(Si = "#F8766D", Ti = "#00BFC4"),
labels = c(expression(S[i]), expression(T[i]))
) +
labs(x = "Parameter", y = "Sobol\u2019 index", fill = "Index") +
theme_bw()